My first floor forced air system is 100k btu for about 800sf. I also have a smaller 2nd floor system in place. The 100k is too large for the space(in NJ) but when I had it tested and "gfixed" regarding duct rerouting, sealing, tuneup etc... the 100k system was still not putting out a high percentage of efficiency. Well below <50% btu's.

My question is, if I ever decide to change out the (now 6 year old) system to a more reasonable sized furnace, would that not present a problem since my ducts are not perfectly ran? They are all in the walls and cannot be moved at this point. Whatever could have been done to fix them within reason HAS been done.

My 100k unit putting out 40k+ seems like a smaller unit would also put out less than the label it gives because of my configuration. Am I thinking correctly here? If I follow the load calc which I did btw, It had a heat loss of 17,000 btu for the first floor. it may not be to my advantage due to the existing duct system.

I'm not ready yet for the new system, but want info so I'm not running around later asking all these questions in desperation, with no heat.

It's also 2.5 tons of AC, btw.

So, is the bottom line system/furnace size fall prey to the weakest link which is my ducts?